Method and apparatus for monitoring the progress of rimming of a steel ingot

ABSTRACT

A physical and/or chemical parameter, the development of which is connected with that of the rimming of an ingot, is measured while the major part of he gases issuing from he ingot in the course of the rimming process escape freely, practically without being disturbed, and while the molten steel has free access to the oxygen of the surrounding atmosphere. The curve representing the variation with time of the value measured is plotted, from which it is possible to obtain indications regarding the development of rimming.

United States Patent [54] METHOD AND APPARATUS FOR MONITORING THE PROGRESS OFRIMMING OF A STEEL INGOT 10 Claims, 7 Drawing Figs.

[52} 11.8. C1 250/833 I-I, 73/19, 250/435 R [51] Int. Cl. G0lj 5/00 [50] Field of Search 250/833 [56] References Cited UNITED STATES PATENTS 2,303,655 12/ 1942 Nutter 73/19 2,582,232 1/1952 Cesaro et a1.. 73/24 2,653,471 9/1953 Clewell 73/24 3,186,232 6/1965 Yates et a1. 73/19 3,283,562 11/1966 Heisig et a1. 73/19 3,286,098 11/1966 Long et a1. 73/24 3,432,288 3/1969 Ardito et a1.... 73/19 3,442,124 5/1969 Warren et al. 73/194 3,483,378 12/1969 Murray 250/83.31R

Primary Examiner-Archie R. Borchelt Attorney-Holman & Stern ABSTRACT: A physical and/or chemical parameter, the development of which is connected with that of the rimming of an ingot, is measured while the major part of he gases issuing from he ingot in the course of the rimming process escape freely, practically without being disturbed, and while the mo]- ten steel has free access to the oxygen of the surrounding atmosphere. The curve representing the variation with time of the value measured is plotted, from which it is possible to obtain indications regarding the development of rimming.

38 AUXILIARY CELL 1 a 28 CONTROL lNFRA-RED 2b 25 .42

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RECORDER 27 lNFRA-RED PATENTED AUG 3 I971 SHEET 1 OF 3 PATENTED we a m SHEET 2 BF 3 38 AUXILIARY CELL t I L CONTROL lNFRA-RED 2b 25 v 42 CELL\\@ 3O 4| I 3| "RECORDER 27 INFRA-RED 36 9 H VOLTAGE TIME PATENTEDAUG 3:971 3.597597 SHEET 3 1F 3 V VOLTAGE VOLTAGE TIME wry me Mam/" fi l/16 50 WRZH U AL METHOD AND APPARATUS FOR MONITORING THE PROGRESS 01F RIMMTNG OF A STEEL INGOT This invention relates to a method and apparatus for monitoring the progress of rimming of steel while it is cooling in the ingot mold.

The deoxidation of nonkilled steel by means of carbon gives rise to the well-known phenomenon of rimming during the course of solidification of the steel in the ingot mould. This phenomenon manifests itself, as is known, by an effervescent effect on the molten top surface of the steel ingot, resulting from a violent release of gas, constituted principally of CO. The effervescence is moreover accompanied by a spattering of incandescent metallic particles.

It is also a known fact that many factors have individual influence on the development of the phenomenon of rimming, i.e. the temperature of the steel and of the ingot mould, the rate of solidification of the metal and its composition, the content of the metal in deoxidizing agents such as Mn, Si, the shape and dimensions of the ingot, and so on.

For some long time it was left to the operator to judge from his own experience by observations the course the rimming was taking. it was customary for the operator to divide the ingots into various classes, on the basis solely of his assessment of the general developments of the rimming process, and relying only on his own, essentially subjective, impressions.

Of course, such a method has serious drawbacks, in particular by reason of the errors which may be made in the assessment, the variations in assessment due to the operator being to a greater or lesser extent fatigued, the nonreproducibility of the measurement results because of the essentially subjective nature of the measurement means, i.e. the eye of the operator, and of the criterion of observation used, i.e. the data assembled in the operators memory from his actual experience.

In order to assess the intensity of the gaseous release in a more objective way, a method has already been proposed for measuring the rate of flow of the gases released during the rimming of an ingot and analyzing them, the said gases being captured by means of a suction hood positioned over the ingot mould during the rimming process. The measurement of the gaseous flow takes place in this connection in a way known per se.

This method, although far more objective than the method outlined above, has nevertheless various drawbacks, among which can be mentioned;

1. It is impossible to observe the actual rimming process since the upper surface of the molten steel is covered by the suction hood.

2. The elimination of the absorption of oxygen from the surrounding atmosphere by the molten steel; this absorption has an important effect on the intensity of the rimming.

The method of the invention has the essential feature that a physical and/or chemical parameter, the development of which is connected with that of the rimming of an ingot, is measured, said measurement being carried out in such a way that the major part of the gases issuing from the ingot in the course of the rimming process can escape freely, practically without being disturbed, and the upper surface of the molten steel has free access to the oxygen of the surrounding atmosphere. The measurement the curve representing is graphed against time to obtain a variation with time of the parameter measured, which indicates the progress of rimming of the steel ingot.

The measurement is preferably carried out for a great number of ingots, for each ingot cast the correlation is plotted, for instance by means of a curve, between the measurement or curve and the variables known to have an influence on the course of the rimming. From statistical observations assembled in this way it is possible to deduce, from knowledge of the curve or of the measurement which has been plotted for each fresh ingot, whatare its characteristics and its type and what modifications have to be made to anv one ofthe parameters influencing the rimming of the ingots so that the curve plotted for a certain ingot comes nearer to or follows a predetermined curve which is considered to be the optimum one.

The statistical observations which itis possible to carry out on the curves which have been plotted can of course have bearing on any of the details of the curves, i.e. their shape, their general course, secondary variations, position of maxima or minimal various slopes and so on.

Numerous methods can be used in accordance with the invention for carrying out any of the measurements likely to give the operator the infonnation he wants on the progress of rimming of the ingot concerned:

1. A sample is taken under standard conditions from the gases escaping from the ingot and the sample is subjected to a quantitative analysis. In this way it is possible to carry out the monitoring of the rimming effect easily, without disturbing the gases released and accordingly without disturbing the phenomenon to be monitored.

This analysis can be carried out in various ways, in particular by observation of the spectrum of emission of the gases of the sample, it being possible from the measurement of the relative intensity of the characteristic rays emitted by the CO and by the CO to determine the CO/CO ratio of the gases as well as the variation taking place over a period. It has moreover been found that the sum of the concentrations of CO and CO plotted in such a sample is proportional to the rate flow of the gases escaping from the metal. By continuously measuring the spectrum of emission of the gases it is accordingly possible to obtain information continuously on the gaseous flow taking place as a result of rimming.

The analysis can alternatively be carried out by observation and measurement of the spectrum of absorption of the escaping from the ingot, this arrangement having the advantage of particularly great accuracy of measurement.

[t is particularly advantageous if the analysis of the gases escaping from the ingot is carried out by measuring the speed of sound in the gases. This method has great simplicity and sensitivity, and by its use it is possible to obtain results which automatically take into account the temperature and the nature of the gases. It is possible moreover to localize automatically the position of the maximum of the curve giving the speed of sound as a function of time, which is important for classifying the types of rimming.

2. The temperature of the gases resulting from the combustion of the CO, evolved during rimming, in the oxygen of the surrounding atmosphere is measured. On the basis of knowledge of the value of the temperature measured, one can obtain information on the rate of evolution of CO from the ingot.

lt has been found advantageous, so as to draw the maximum useful indications form the above-mentioned measurements, to standardize the operation conditions in which the observations are made. This standardization has particular reference to the locality where the measurement selected is carried out, as well as to the measurement devices used and the way in which they areused. Accordingly, results obtained by means of measurements carried out under these same operating conditions will be fully comparable.

The measurement of the temperature of the said combustion gases can be carried out in many different ways, for instance pyrometrically, acoustically, by measurement of the intensity of continuous radiation emitted by the gases (this corresponds to the phenomenon of the combustion of CO), or by measurement of the specific radiation of the atoms entrained by the gaseous release. It has been found that there. is a welldefined correlation between the temperature of the gases resulting from the combustion of the CO, evolved during rimming, in the oxygen of the surrounding air and the various acoustical or radiation measurements enumerated above.

3. The characteristics of the flux of solid particles (sparks) emitted by the ingot as it solidifies are measured.

This embodiment is based on the consideration that the quantity of sparks. and conseouentlv their radiation. emitted by the ingot during a given period is connected with the rate of flow of gas escaping from the ingot during this period. In accordance with this embodiment, various measurements can be effected on the particles to determine the quantity of sparks, among these measurements the most interesting, by reason of their rapidity, accuracy and facility, are measurements of the radiation emitted by the particles (in the band of lg to 2p).

Among other measures which are equally useful there can be mentioned that of counting or weighing the particles emitted during a certain period.

4. The thennal radiation emitted by the ingot as it solidifies in the ingot mould is recorded. Use is made ofa radiation capturing device, hereinafter referred to as the main capturing device, which is sensitive to visible and/or infrared radiation. The main capturing device is focused on the region located just above the level of the steel, where the emission of radiation occurs by incandescence and/or luminescence coming from the ingot as it solidifies in the ingot mould. By means ofa second radiation capturing device, hereinafter referred to as the secondary capturing device, the field of observation of which is substantially narrower than that ofthe main capturing device, the teeming jet is sighted, this making it possible to determine the exact moment when teeming ends and consequently to determine the exact moment when a curve traced by a graph generator connected to the output of the main capturing device is no longer disturbed by the thermal radiation due to the teeming. The setting off of the graph generator connected to the main radiation capturing device may be made dependent on the disappearance of the signal which, in the circuit of the secondary capturing device, corresponds to the presence of the teeming jet in the field of observation of the secondary capturing device, the disappearance of the signal being utilized by means known per se to put the graph generator into operation automatically.

5. During rimming, the whole or a portion ofthe sonic or ultrasonic spectrum of the sound vibrations coming from the ingot is measured. It has been found also that there is a cor relation between the development of the phenomenon of rimming observed and that of its sound spectrum, and as a result that it is possible to draw conclusions on the former by means of measurements carried out on the latter.

The invention further consists in apparatus for monitoring the progress of rimming of a steel ingot, which comprises a radiation capturing device sensitive to thermal radiation whose field of observation is restricted by the aperture of a suitable divergent flaring opening or horn. The capturing device is arranged so as to be substantially coaxial to the axis of the flaring opening. The output of the capturing device is connected to a graph generator to plot a curve giving as a function of time a correlative value of the electric voltage ap pearing at the terminals of the main capturing device. The capturing device is mounted on a support making it possible to adjust the capturing device in height, direction and/or angle of inclination.

The apparatus further comprises a thermal radiation detector, whose field of observation is substantially narrower than that of the main capturing device. The output of the detector controls the graph generator operation.

.This apparatus has the advantage that by it it is possible to determine the exact moment when teeming has terminated and to put the graph generator in operation at this exact moment. In this way any disturbance of the main signal due to the presence of the teeming jet is suppressed, since the signal is recorded only when teeming has terminated. No error of interpretation can be produced due to the teeming jet in examining the curves recorded.

Ofcourse it is possible to arrange the radiation detector at a place other than above the main capturing device, without thereby going beyond the scope ofthe invention. The essential thing is that the field of observation of the main capturing device incorporates practically the whole width of the shower of sparks coming from the ingot mould, whereas the detector needs to be able to focus only the teeming jet.

In one advantageous embodiment of the apparatus:

l. The output of the main capturing device is connected in one arm of a Wheatstone bridge, and a capturing device, as similar as possible to the main capturing device, but provided with a screen adapted to mask it from the radiations to which the main capturing device is subjected, is connected in the other arm.

2. The graph generator is connected in the diagonal of the Wheatstone bridge.

By this embodiment it is possible to eliminate the influence of any current circulating in the main capturing device not due to the radiations to be observed. As the bridge assembly can realize automatic equilibrium of its arms in respect of such current, the sensitivity of the measurement is thereby clearly increased.

The accompanying drawings, which are not to scale and are given by way of example only and in no restrictive sense, illustrate en embodiment of the invention in the case where use is made of cells which are sensitive to infrared radiation.

In the drawings:

FIG. 1 is a diagrammatic elevation of a form of apparatus for carrying out one embodiment of the invention;

FIG. la is a diagrammatic side elevation of part of the apparatus of FIG. 1;

FIG. lb is a plan view of the apparatus of FIG. la;

FIG. 2 is a circuit diagram illustrating the processing of the readings obtained by the apparatus of FIG. 1; and

FIGS. 3, 4 and 5 are curves showing different types of results obtained.

Referring to FIGS 1, la and lb, a tripod made of steel is constituted of three supports 1, 2, 3 connected at I20 to a central sleeve 4 which has a verticalaxis 5 and is used as pivoting member for a vertical column 6 supporting the observation apparatus. The column 6 is fixed in the sleeve 4 by means of a clamping screw 7 and is provided at its upper end with a sleeve 8 which can be tightened onto the column 6 by means ofa set of suitable screws 9. The sleeve 8 is welded to a plate 10 which has a pivot 11 about which a plate 12, which is integral with the body of the cylindrical sighting member 13, can rotate. A circular slide 14, formed in the plate 12, and a screw 15, makes it possible to fix the cylindrical sighting member 13 and incline it at an angle.

The sighting and observation member of the apparatus is constituted of the optical cylindrical member 13 provided at one end with an observation device 18 which is sensitive to infrared radiation, screwed at 17 coaxially to the said cylindrical member 13. The other end 20 is attached to a flaring opening 19 or horn ofdivergent triangular shape and rectangular cross section. The internal surface of the flaring opening 19 is black in color and unpolished, in order to avoid an parasitic reflection, and its angular divergence is in relation to the directional diagram of the sensitive device 13. The end 21 of the flaring opening has regulable shutters 22, 23 by which it is possible to adjust the desired value for the width ofthe sighting gap 24.

Constituted in thisway, the sighting member can be orientated at a certain angle and arranged at a certain distance from the ingot mould in which is located the steel, the conventional measurement of the rimming of which it is desired to observe and record.

The device 18 sensitive to infrared radiation is composed of (FIG. 2) two identical cells 25 and 27 which are sensitive to infrared radiation and are located in two different branches of a measurement bridge. The cell 25 is masked by a hood 26 which prevents any external radiation from reaching it. The other cell 27 is the only one which is subjected to radiation coming from the rimming steel.

The measurement bridge has moreover two identical resistors 28 and 29, each having a slider 30, 31 between which the measurement and recording apparatus 32 is attached. Direct current is supplied at 33, 34, and the two sliders 30 and 31 are mechanically rigid with each other, in such a way that the position of the slider 30 in respect of the resistor 28 is always the same as that of the slider 31 in respect of the resistor 29, the two sliders being displaced always simultaneously in the same way and in the same direction.

In order to reduce fluctuations of the current passing through the recorder, two variable capacitors 3 5, 36 are ar-I same control member 37, are used to damp the resistance cir-.

cuit 28, 29.

The indications plotted by the recorder 32 are representative solely of the intensity of infrared radiation received by the cell 27, since any parasitic influence of ambient temperature has been eliminated by virtue of the cell 25 which is provided with the hood 26.

So that the device of the invention shall be able to supply indications which, as recording begins, are not affected by the infrared emission due to the presence of the casting jet, an auxiliary cell 38 is arranged a little above the cylindrical sighting member 17, and its optical axis can be arranged in such a way that the cell 38 is sensitive only to the radiation coming from the teeming jet.

The auxiliary cell 38 is fixed to the flaring opening 19 by conventional means, and by it it is possible to sight the teem ing jet before it passes in front of the observation field of the device 18. The current passing in the circuit of the cell 38 energizes by means of a suitable control member 40 a relay 42, which by means of the switch 41 blocks the supply of current to the recording device as long as the teeming jet influences the cell 38. Thus the operation of the recorder starts automatically as soon as the teeming jet has effectively ceased. Furthermore, so as to be able to begin the recording as soon as the teeming jet has ceased, the indicator pointer of the recorder 32 has been kept by artificial means at full scale deflection. As a result it is not necessary to wait until the pointer has passed through the entire scale before being able to begin the tracing of the diagram envisaged; it is known that .in actual fact the diagrams to be recorded are always decreasing at the beginning of the recording.

From the typical recording diagrams given below in FIGS. 3, 4 and 5 it will be clear how use can be made of the indications plotted by means of the apparatus of the invention.

The diagrams show. the voltage applied to the recorder as ordinate, and the time which has passed since the beginning of casting as abscissa. The total duration of observation of the rimming effect is approximately 15 minutes in the cases envisaged.

The first diagram (FIG. 3) represents the course of rimming of an ingot whose structure can be considered as satisfactory. Several constituent parts can be distinguished therein.

I. Teeming is taking place continuously and the apparatus is recording the heat emission due to the teeming jet.

ll. Teeming has just ceased. At this exact moment radiation is at its most intense, accordingly the curve records a maximum. The rimming has started up normally; a rapid though regular drop in intensity is observed, which corresponds to a decrease in the release of gas, possibly accompanied byafall in the level of the steel in the ingot mould. The ring of solidification of the steel forms uniformlyand regularlyand advances horizontally towards the center of the ingot mould.

III. The rimming continues to decrease but more and more slowly. On the other hand, the solidification ring becomes more and more contracted, decreasing the liquid portion of the head of the ingot emitting the sparks.

IV. End of observation.

An ingot having a rimming curve analogous to that of FIG. 3 has a surface structure which has no subcutaneous blowholes or pitting. There is generally no need to modify the operations on subsequent castings of steel from the same refining operations.

The second curve (FIG. 4) illustrates the course of the rimming of an ingot with closed blowholes having a head of which the appearance necessitates considerable rejection of the steel. Here again, there are several constituent parts:

I. Teeming is continuing all the time and the apparatus records the radiant heat of the teeming jet.

ll. Teeming ceases, the emission passes through maximum intensity, and rimming'starts. The beginning of the rimming phenomenon is nevertheless quite slow, the developing ring forms badly and the movement of the steel resulting from the rimming phenomenon, though it began satisfactorily, becomes disorientated.

, Ill. The ring continues to form badly. The release of gas is not active enough to prevent blowholes being imprisoned in the solidified steel; this causes the liquid steel to rise up again in the ingot mould resulting in an increase in the intensity of infrared radiation recorded by the cell. The ring rises up again, with the liquid steel continually overflowing it,resulting in poor quality closure of the ingot head.

IV. The signal decreases by reason of the considerable amount of scum floating on the surface of the ingot, with the result that the normal formation of sparks is prevented.

V. Observation is terminated.

The conclusion to be drawn from observing a diagram of this type is the insufficiency of the level of oxidation of the ingot. If the same thing happens for a number of heats, the furnace charges will need to be recalculated and also the moment when conversion is stopped, so as to raise the level of oxygen of the refined finished steel. It is even possible if necessary to correct the oxygen insufficiency of the ingots which have not yet been cast, on the basis of observations carried out on the first ingot or ingots teemed.

The third diagram (FIG. 5) concerns an ingot where the rimming phenomenon does not start up immediately. At the termination of teeming I, the level of the steel rises suddenly to a considerable extent; there then follows a series of descending and ascending movements of great amplitude, during the course of which the average level of the steel passes through a maximum, then starts to drop, following a curve which recalls the stages [I and II of FIG. 3, with the amplitude of the fluctuations continuing to decrease. In the course of this stage I" there is vigorous rimming and the ring which forms is very substantially horizontal. Rimming then stops, in a normal, gradual way, in stage IV. Here, the ingot surface is defaced by the presence of pitting due to excess oxygen, the blemish appearing over the entire surface of the ingot. It is necessary to correct the deoxidation state of the metal, either in the ladle at the moment when the converter contents are emptied into the ladle, or in the ingot mould at the moment of teeming, for instance by adding thereto a certain amount of aluminum.

To facilitate assessment regarding any modifications to be made in subsequent castings, it is possible to quantify adequately the curves recorded in accordance with the method of the invention. It is possible in this way to characterize the curves recorded by means of a numerical value indicative of any modifications to be made to subsequent castings.

Various modifications may be made within the scope of the invention.

We claim:

l. A method of monitoring the progress of rimming of a steel ingot, comprising the steps of continuously measuring the intensity of thermal radiation emitted from a region located just above the surface of the steel, by means of a cell sensitive to thermal radiation, the major part of the gases evolved during rimming escaping freely, the upper surface of the molten steel having free access to the oxygen of the atmosphere; continuously transmitting a signal representing said intensity to a graph generator; and tracing a graph of the signal against time, whereby a curve is obtained representing the progress of rimming of the steel ingot.

2. A method as claimed in claim 1, including the steps of carrying out the continuous measuring step for each of a plurality of ingots during rimming, graphing the measurement against time during rimming of each ingot, relating the curves obtained to variables known to influence the progress of rimming and to the quality of the ingots obtained, carrying out the continuous measuring step for a further ingot during rimming, graphing the measurement against time during rimming of said further ingot, and comparing the curve obtained for said further ingot with the curves related to said variables and said ingot quality.

3. A method as claimed in claim 1, including during teeming of the ingot, selectively measuring the intensity of thermal radiation emitted by the teeming jet to determine the moment at which the jet ceases to affect said curve.

4. A method as claimed in claim 3, including automatically starting the tracing step on cessation of thermal radiation due to the teemingjet.

5. A method as claimed in claim 1, wherein the measuring step comprises i. receiving the thermal radiation by means ofa first cell sensitive to thermal radiation;

ii. arranging a second cell substantially similar to the first cell at a position adjacent to the first cell;

iii. shielding the second cell from the thermal radiation emitted from the region located just above the surface of the steel; and

iv. transmitting the output of the first cell and the output of the second cell to a comparison circuit providing the signal representing the intensity.

6. A method as claimed in claim 1, including carrying out the measuring, transmitting, and tracing steps for a plurality of ingots, and when the curve obtained for a given ingot is unsatisfactory, modifying at least one of the variables known to influence rimming so as to cause curves obtained for succeeding ingots to approach an ideal curve.

7. Apparatus for monitoring the progress of rimming of a steel ingot, comprising a. means for measuring the intensity of thermal radiation coming from a given region, the measuring means including a cell sensitive to thermal radiation and means for restricting the field of observation ofthe cell;

b. a graph generator connected to the output of the measuring means, to trace a graph of the intensity measurement against time;

c. a thermal radiation detector for sighting a teeming jet, having a field of observation narrower than said cell; and

d. a relay controlling the graph generator, the relay being connected to the output of the thermal radiation detector.

8. Apparatus as claimed in claim 7, wherein the observation field restricting means comprises a divergent horn of rectangular transverse section, the longitudinal axis of the horn coinciding substantially with the radiation reception axis of the cell.

9. Apparatus as claimed in claim 7, wherein the measuring means comprises i. a first cell sensitive to thermal radiation;

ii. means for restricting the field of observation of the first cell to a given region;

iii. a second cell substantially similar to the first cell;

iv. a shield protecting the second cell from thermal radiation coming from points in the field of observation of the first cell; and

v. a comparison circuit connected to the output of the first cell and the output of the second cell to provide a signal proportional to the difference between the signals received from the outputs of the first and second cells, the output of the comparison circuit being connected to the graph generator.

10. Apparatus as claimed in claim 9, wherein the comparison circuit comprises a Wheatstone bridge circuit, the output of the first cell being connected in one arm of the bridge, the output of the second cell being connected in the other arm of the bridge, and the graph generator being connected in the diagonal of the bridge. 

1. A method of monitoring the progress of rimming of a steel ingot, comprising the steps of continuously measuring the intensity of thermal radiation emitted from a region located just above the surface of the steel, by means of a cell sensitive to thermal radiation, the major part of the gases evolved during rimming escaping freely, the upper surface of the molten steel having free access to the oxygen of the atmosphere; continuously transmitting a signal representing said intensity to a graph generator; and tracing a graph of the signal against time, whereby a curve is obtained representing the progress of rimming of the steel ingot.
 2. A method as claimed in claim 1, including the steps of carrying out the continuous measuring step for each of a plurality of ingots during rimming, graphing the measurement against time during rimming of each ingot, relating the curves obtained to variables known to influence the progress of rimming and to the quality of the ingots obtained, carrying out the continuous measuring step for a further ingot during rimming, graphing the measurement against time during rimming of said further ingot, and comparing the curve obtained for said further ingot with the curves related to said variables and said ingot quality.
 3. A method as claimed in claim 1, including during teeming of the ingot, selectively measuring the intensity of thermal radiation emitted by the teeming jet to determine the moment at which the jet ceases to affect said curve.
 4. A method as claimed in claim 3, including automatically starting the tracing step on cessation of thermal radiation due to the teeming jet.
 5. A method as claimed in claim 1, wherein the measuring step comprises i. receiving the thermal radiation by means of a first cell sensitive to thermal radiation; ii. arranging a second cell substantially similar to the first cell at a position adjacent to the first cell; iii. shielding the second cell from the thermal radiation emitted from the region located just above the surface of the steel; and iv. transmitting the output of the first cell and the output of the second cell to a comparison circuit providing the signal representing the intensity.
 6. A method as claimed in claim 1, including carrying out the measuring, transmitting, and tracing steps for a plurality of ingots, and when the curve obtained for a given ingot is unsatisfactory, modifying at least one of the variables known to influence rimming so as to cause curves obtained for succeeding ingots to approach an ideal curve.
 7. Apparatus for monitoring the progress of rimming of a steel ingot, compriSing a. means for measuring the intensity of thermal radiation coming from a given region, the measuring means including a cell sensitive to thermal radiation and means for restricting the field of observation of the cell; b. a graph generator connected to the output of the measuring means, to trace a graph of the intensity measurement against time; c. a thermal radiation detector for sighting a teeming jet, having a field of observation narrower than said cell; and d. a relay controlling the graph generator, the relay being connected to the output of the thermal radiation detector.
 8. Apparatus as claimed in claim 7, wherein the observation field restricting means comprises a divergent horn of rectangular transverse section, the longitudinal axis of the horn coinciding substantially with the radiation reception axis of the cell.
 9. Apparatus as claimed in claim 7, wherein the measuring means comprises i. a first cell sensitive to thermal radiation; ii. means for restricting the field of observation of the first cell to a given region; iii. a second cell substantially similar to the first cell; iv. a shield protecting the second cell from thermal radiation coming from points in the field of observation of the first cell; and v. a comparison circuit connected to the output of the first cell and the output of the second cell to provide a signal proportional to the difference between the signals received from the outputs of the first and second cells, the output of the comparison circuit being connected to the graph generator.
 10. Apparatus as claimed in claim 9, wherein the comparison circuit comprises a Wheatstone bridge circuit, the output of the first cell being connected in one arm of the bridge, the output of the second cell being connected in the other arm of the bridge, and the graph generator being connected in the diagonal of the bridge. 